Objectives: To describe and interpret the ultrastructure of selected pathologic states and to compare the results with suitable controls. To look for and study the inciting agent or accompanying agent in neoplastic and other disease states using human, animal and tissue culture. Major findings: Further information on the dense granule of the pox viruses was obtained and some of the results reported at the 8th International Congress on Electron Microscopy in Canberra, Australia. The pox viruses studied could be divided into three statistically significant groups on the basis of the size of the dense granule present in the mature virus particle. It was also found that the dense granule was significantly smaller in the mature forms than in the developmental forms or than in the forms undergoing uncoating. Attempts at characterizing its chemical nature have not been successful. Quantimet measurements of area were compared with the same measurements of area using a measuring magnifier graduated in tenths of millimeters. The quantimet values were significantly smaller in most instances. A cylindroid lamella-particle complex in malignant lymphoma of northern pike has been studied further in collaboration with Dr. Dawe. This structure is similar to those described in hairy cell leukemia. Proposed course: A major effort will be made to characterize the granule and cylindroid lamella particle complex chemically by histochemical techniques and to assess the latter's possible relation to the structure in "hairy cell" leukemia.